With that model in mind, the goal then could be a "minimum operational nuclear capability" of 80 to 100 nuclear missiles, including some that could reach the United States, Hahm estimated. The weapons would be hidden around the country to prevent detection, in caves, tunnels, amid conventional missile stockpiles, in dense population centers and on mobile launchers, Hahm said in an interview. He speculated that such an effort could take five to 10 years.
One hundred warheads in five years is probably alarmist, Matthew Kroenig, a nuclear expert at Georgetown University, wrote in an email, but "it would be naive to assume that Pyongyang will keep a small and primitive arsenal forever. Rather, it is likely that they will rapidly move to expand their arsenal and means of delivery."
Many analysts believe it has taken so long to come to terms with North Korea's intentions because of a long history of chronic underestimation.
This may stem from the North's poverty — it has a GDP rivaling Senegal — or from the images of goose-stepping soldiers and leadership-worshipping masses that can seem to foreigners to be frozen in the amber of Cold War stereotype.
"It's not a pretend nuclear-strike capability," said John Delury, an analyst at Seoul's Yonsei University. "We're past that point where you can just laugh it off."
Some of North Korea's recent threats were widely dismissed, including vows of nuclear strikes on Washington and Seoul. But another announcement was more worrying: It promised to restart all nuclear fuel production, including a mothballed reactor that could eventually make a bomb's worth of plutonium annually. Estimates on how long it would take to restart plutonium facilities vary from three months to a year.
An even bigger fear is a uranium enrichment program unveiled in late 2010 that could provide a second, more easily concealed source of bomb fuel, quickly augmenting Pyongyang's arsenal, Klingner, the former U.S. intelligence official, said in an interview in Seoul.
"We've underestimated them," said Klingner, now an expert with the conservative Heritage Foundation think tank in Washington. "People made fun of their long-range missile until it didn't fail. There was sort of this, 'Why wasn't I informed there was a long-range missile threat?' Well, we've been warning you for 15 years."
North Korea has yet to prove it possesses an arsenal of flight-capable, nuclear-tipped missiles. But in a statement that still reverberates, then-U.S. Defense Secretary Robert Gates said in early 2011 that Pyongyang will have a limited ability to deliver a weapon to U.S. shores within five years using intercontinental ballistic missiles.
To Lewis, the Monterey Institute expert, the North's December rocket launch clearly showed what it can do. "If you think North Korea lacks the basic capabilities to build long-range missiles, guess again. A space launcher is not exactly the same thing, but it's most of the way there," he said.
What to do is as tough to determine now as it was when President Bill Clinton considered bombing a North Korean reactor in 1994, when worries over North Korea's nuclear ambitions and its refusal to admit U.N. inspectors sparked the first North Korean nuclear crisis. No strikes were ordered. Former President Jimmy Carter visited Pyongyang and diplomats later struck a deal, now defunct, to freeze Pyongyang's nuclear program in exchange for U.S. aid.
Campbell, the former U.S. diplomat, said mounting "serial provocations" from Pyongyang have "caused a quiet rethinking in a variety of capitals about just how difficult it is to construct any engagement strategy with North Korea."
For many, the key is China, which is Pyongyang's economic and diplomatic lifeline, providing nearly all of its fuel and most of its trade.
A frustrated new Chinese leadership has recently displayed willingness to work with Washington to apply pressure on Pyongyang. But Beijing's primary goal is stability. A crisis could damage its economy; a collapse in Pyongyang could push refugees into China or leave a U.S.-friendly unified Korea on its borders.
Campbell said Washington has told Beijing "that if this process continues, we will be taking defensive and other steps in Northeast Asia that will not be in China's strategic interests."
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